Method of drop-forging sintered workpieces

ABSTRACT

The present drop-forging method of presintered workpieces employes a drop-forging die which has a dimension transverse to the direction of motion of a die member larger than the dimension of a preshaped blank. The shape of the blank is selected to deviate from the shape of the drop-forging die to such an extent as to assure a massive deformation in the following drop-forging step.

llnited States atet 1191 Sehober Sept. 3, 1974 [54] METHQK) 013 DROP-FORGENG SINTERE 2,778,064 1/l957 Clark 72/360 3,069,756 12/1962 Colestock 29/1592 WORKPIECES 3,355,930 12/1967 Fedorov 29/1592 X Inventor: Franz Sclwber, 3,561,087 2/1971 Koehler 29/4205 x Muenchen-Obermenzmg, Germany 3,785,038 1/1974 Zapf 29/4205 [73] Assignee: Bayerisches Leichtmetallwerk Graf FOREIGN PATENTS OR APPLICATIONS a s g g Wahlsmii 1,433,073 l/1969 Germany 29 4205 umc ermany OTHER PUBLICATIONS [22] led: 1973 AFC. 191, 168, F Singer, published 4/27/1943. [21] Appl. No.: 350,802

Primary Examiner-C. W. Lanham Assistant ExaminerD. C. Reiley, III [30] Foreign Application Pnomy Data Attorney, Agent, or Firm-Wolfgang G. Fasse; Willard Apr. 22, 1972 Germany 2219856 W Roberts [52] U.S. Cl 29/1592, 29/4205, 29/DI7(;./3l68O, [57] ABSTRACT 511 1111. c1 821k 1/30 h Present drop-forging e of piesimered W [58] Field of Search..... 29/4205, DIG. 18, DIG. 31, 9 employ a Q E d1e i' 29/592; 72/360 s1on transverse to the d1rect1on of motlon of a d1e member larger than the dimension of a preshaped [56] References Cited blank. The shape of the blank is selected to deviate I from the shape of the drop-forging die to such an ex- 2 285 575 ZZ PATENTS 29/159 2 UX tent as to assure a massive deformation in the followertz 2,373,405 4/1945 Lowit 29/DIG. 31 mg drop forgmg Step. 2,494,935 1/1950 Dunn 72/360 10 Claims, 3 Drawing Figures PATENTEUSEPB 1914 I 3,832,763

SHEET 1 [IF 2 I III I I 3" WA A w METHOD OF DROP-FORGING SINTERED WORKPEECES BACKGROUND OF THE INVENTION The invention relates to a method of drop-forging sintered workpieces wherein a sintered preshaped blank is form pressed in a die the dimensions of which, in a direction transverse to the direction of movement of a die member, are larger than the dimensions of the blank.

It is well known that sintered workpieces cannot be easily densified to a high degree. The common density of sintered workpieces is about 85 90 percent of the theoretical value. Usually a sintered workpiece is pressed in two successive pressing or molding steps whereby after the first pressing step the workpiece is annealed or presintered. The result is a workpiece with a density of more than 90 percent of the theoretical density.

According to another known method described in British Pat. No. 1,256,763, a sintered blank is pressed within a mold which is larger in a direction transverse to the movement of a pressing die member than the transverse-dimensions of the blank. During said pressing process the material is displaced in a direction transverse to the movement of the pressing die member. By the use of several such pressing steps, each step, if desired, may be followed by an intermediate sintering process, whereby a density of up to 99.9 percent of the theoretical density may be attained. According to said known method, the maximum density is attained only by several successive pressing processes, which should be followed by further sinter or annealing processes and by an additional calibrating step, if desired a forging step. Thus, according to said known process, it is attempted to progressively adapt the density of the sintered workpiece to the theoretical density by way of a step by step deformation.

OBJECTS OF THE INVENTION In view of the foregoing, it is the aim of the invention to achieve the following objects singly or in combination:

to densify sintered workpieces substantially in one densification step to about 100 percent of the theoretical density whereby the workpieces simultaneously receive their final shape with high precision;

to provide a drop-forging process which will control the material flow in such a manner that the material flows first in an outward direction and substantially transversely to the direction of movement of a die member whereupon the material flows substantially in parallel to the direction of die movement;

to achieve in the drop-forging of sintered, preshaped blanks a high precision combined with a high surface quality of the finished product, as well as a predetermined grain flow inside the sintered, drop-forged finished product;

to avoid inside the workpiece regions with smaller density or rather to assure a uniform density throughout the entire workpiece; and

to avoid after machining or surface improvement work by achieving the desired surface qualities and precise dimensions of the workpiece.

SUMMARY OF THE INVENTION According to the invention, the presintered blank is drop-forged in a die whereby the shape of said blank differs from the shape of the die to such a degree that during the drop-forging a massive deformation takes place.

According to the above mentioned known method, a genuine massive deformation is not achieved. Contrary thereto, according to the invention, the blank is intentionally deformed to such an extent that the material is subjected to a substantial flow whereby it is displaced during the drop forging. Thus, not only the desired high density and the high precision are attained but moreover a special grain flow is realized within the product, which grain flow corresponds to that achieved in common (non-sintered) drop-forged workpieces. According to the method of the invention the blank is deformed to a high degree whereby surprisingly a material flow or displacement of material is achieved which heretofore seemed to be impossible in connection with the producing of sintered workpieces.

The drop forging of the sintered workpieces according to the invention may be accomplished in a cold or warm manner. If desired, even forging by explosion may be used. The respective forging method will be selected with regard to the type of material or the desired final condition of the sintered workpiece.

The flow of material during the massive deformation may be controlled according to the invention in such a way that a certain grain flow is attained. For instance, for the control of the material flow, the blank may be heated to locally differing degrees prior to the warm forging step. Thus, the material heated in a higher degree is subjected to flow first.

Another advantageous possibility to control the material flow is to adapt the shape of the blank to the shape of the sintered workpiece in such a way that during the forging the material is flowing in a predetermined direction. If desired, the shape of the blank may be combined with the feature of locally heating different regions of the blank to different temperatures.

BRIEF FIGURE DESCRIPTION In order that the invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein:

FIG. 1 illustrates a drop-forging die with opened die members partially broken away whereby a sintered blank is located inside the lower die member prior to the drop-forging step;

FIG. 2 shows the die and the blank according to FIG. I after the drop-forging step; and

FIG. 3 shows the finished sintered, drop-forged workpiece produced by the forging process resulting in the position of the die members as shown in FIG. 2.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS The production of a bevel gear wheel made of sintered material according to the invention will be described with reference to FIGS. 1 to 3. As shown in FIG. 1, the presintered blank 3 located in the lower die member 1 deviates in its shape considerably from the final shape shown in FIG. 3 as determined by the lower die member l and by the upper die member 2. The blank 3 is of cylindrical shape and has a flow facilitating frustum 11 at its side situated in the lower die member 1.

The portion 4 of the die for forming the teeth of the bevel gear is shaped in the lower die member ll, whereas the remaining peripheral part of the bevel gear wheel and the adjacent frontface are formed by the upper die member 2. A cylindrical lower punch is slidably supported in the lower die member 1. The upper die member 2 comprises a center thorn 6 having the shape of a truncated cone or frustum projecting downwardly into the die cavity whereby said conediameter decreases downwardly. As shown in the drawings, especially in FIG. 2, the frontface of the lower punch 5 facing into the die cavity is smaller than the frontfacc of the center thorn 6 also facing to the die cavity.

Thus a higher pressure arises at the frontface of the lower punch 5 than at the frontface of the center thorn 6 when the die members 1 and 2 are moved toward each other. As the closing continues, the material of the blank is forced to flow first in the range of the lower punch 5 and the material of the blank 3 flows in direction indicated by the arrows A from the inside to the outside and into the toothed portion 4 of the lower die member 1.

As the closing of the die members 1 and 2 progresses, the surface in contact with the flowing blank 3 at the lower die member keeps increasing until the surface pressure at the frontface of the center thorn 6 of the upper die member 2 increases to the point where the blank begins to flow also in its upper region whereby the material is displaced into the cavity 7 of the upper die member 2. The material which so far has been displaced in the direction of the arrow A is deflected at the conical periphery of the lower die member 1 and now follows the arrow B and thus flows substantially in parallel to the directiion of the teeth. The material is thus forced to flow outwardly and upwardly.

During the closing process of the die members 1 and 2, the lower punch 5 is displaced into the die cavity whereby it progressively enters the mid-region of the blank 3 just as the center thorn 6, whereby the entire material of the blank 3, except a remaining web 8, shown in FIGS. 2 and 3, is displaced from the inside outwardly and in an upward direction B.

The above described material flow illustrates what is meant by a substantial formation enabling the production of a bevel gear wheel 9 of very precise shape as shown in FIG. 3. The finished wheel need not be reworked within the region of the teeth 10. The bevel gear wheel 9 has high strength, density and tenacity without the need of a step by step deformation.

Especially good results are obtained according to the invention in that the material during the forging is completely displaced from the inner or mid-region of the blank toward the outer region of the die because the density and tenacity which may be achieved is increased considerably by this displacement since the densifying takes place not only from the outside but also from inside. According to this preferred embodiment, it is possible to produce, for instance, high precision gear wheels having an axial bore, whereby a reworking of the toothed flanks is completely obviated. Toothed gear wheels of sinter material forged as described above are not only of very high strength and density but also of very high tenacity, whereby it is possible to obtain an excellent control of the distribution of the strength characteristics by a controlled flow of material.

Another advantage of the invention is seen in that due to the massive deformation of the blank the occurence of areas of lower density, especially in the internal regions of the workpiece, is avoided.

Further, due to the material flow a uniform pressure distribution is obtained within the die, thus avoiding local maximum pressures or shocks in the die whereby the life of the die has been substantially extended while sintered, forged pieces of high strength are being produced by the method according to the invention. The flow process causes an additional strengthening, a high tenacity and altogether an improvement of the mechanical characteristics as compared to conventional sintered workpieces.

It is an important step of the invention to apply the advantages obtainable by the so called precision or exact-forging-process, for instance high shape precision, surface quality and grain flow, to sintered workpieces.

It is surprising that according to the invention a forged sintered workpiece can be obtained, which with regard to its tenacity characteristics it is comparable to forged workpieces produced of materials by metallurgical melting processes, and that the present sintered workpieces have strength characteristics substantially superior to the respective characteristics of conventional sintered workpieces.

Additionally, the invention avoids an extra sintering step after the drop-forging because the forging process itself results in the desired high density and in the other mechanical characteristics to a full extent.

The invention uses a powder metal blank produced by compression molding from a homogeneous powder whereby the blank is sintered before the forging to a density of more than percent of the theoretically obtainable density. Thus it is possible that the blank on being massively deformed does not decompose into its elements, whereby the flow of material would be disturbed.

A blank made of powder mixtures of different alloying elements is sintered to such degree prior to the forging that the diffusion of the alloy elements of the material takes place substantially during said sintering. If the blank is to be subsequently worked by warm forging, care is to be taken that the alloying process is completed at the latest during the preheating to the forging temperature.

If desired, the finished sintered workpiece produced by forging according to the invention may be subjected to an after treatment, for example it may be tempered.

In an example for producing bevel gear wheels, cylindrical blanks 3 where predensified and sintered up to 80 percent of the theoretically obtainable final density. The sintering temperature was between about 1 and 1200C, whereby an analytically homogeneous sinter-alloy was produced. Afterwards, the sintered blanks, in the presence of an inert gas, were heated to a forging temperature of about 1l80 to 1250C depending on the composition of the material, whereby a sufficient diffusion of the alloy-components is assured. By the following forging process with a forging pressure of more than 10 tons/cm precisely shaped conical toothed wheels were produced whereby without further sintering a final density of practically 100 percent was obtained.

The term massive deformation as used above means that the deformation caused by the drop forging results in a density of the drop-forged product which corresponds substantially to 100 percent of the theoretically possible density. Actual dimensional ranges for the blank and for the finished product will vary widely depending on the final shape of the drop-forged product. However, as long as the density of the final product approaches substantially 100 percent a massive deformation is said to have been accomplished. In one example the blank 3 shown in the drawings had a diameter of 63 mm, a height of 91 mm and a 30 bevel at the lower end. This blank was massively deformed as taught by the invention in a single dropforging step to assume the final shape of a bevel gear as shown in FIG. 3, whereby the diameter of the output tip of the teeth was 128 mm and the final height of the gear was 40.5 mm. The upper cavity had a diameter of 38 mm and a depth of 17 mm while the lower, downwardly facing cavity had a diameter of 28 mm and a depth of mm. It is surprising that these massive deformations with controlled material flow can be achieved in a single drop forging step.

Although the invention has been described with reference to specific examples, it is to be understood that it is intended to cover all modifications and equivalents within the scope of the appended claims.

What is claimed is:

1. A method for producing a drop forged sintered workpiece from a pre-pressed sintered body in which the body is deformed by upsetting to such a degree that the material of the body begins to flow; comprising placing a solid cylindrical sintered pre-pressed body into a cavity of a forging die, the cavity having a lateral dimension normal to the forging direction that is larger than the diameter of said body, and substantially completely pressing the material of the sintered body in a single step out of the central axial portion thereof to flow into the circumferential region of said die.

2. A method for producing a drop forged sintered workpiece from a pre-pressed sintered body in which the body is deformed by upsetting by such a degree that the material of the body begins to flow; comprising placing a first end of a solid cylindrical sintered prepressed body onto a central core of the cavity of a first forging die, the cavity having a lateral dimension normal to the forging direction and axis of the body that is larger than the direction of said body, and substantially completely pressing the material of said body in a single step out of the central axial portion thereof to flow into the circumferential region of said first die by pressing the other end of said body with a second forging die.

3. The method of claim 2 for forming a gear, in which said cylindrical body has a flow cone at one end, and said circumferential region of said first die is formed with teeth for forming the teeth of said gear, wherein said step of placing comprises placing said body in said first die with said flow cone facing said first die,

whereby said body will flow primarily radially out wardly and toward said second die insaid pressing step.

4. The method of claim 2, in which said second die has a central core with a cross section smaller than the cross section of the core of said first die, wherein said step of pressing the other end of said body comprises pressing the other end of said body with said central core of said second die.

5. The method of claim 2, in which said central core comprises a punch in said first die displaceable into said second die, wherein said step of pressing comprises continuously pushing said punch into said cavity of said first die.

6. The method of claim 2, comprising locally preheating said body prior to said pressing step whereby different portions of said body have different temperatures, to control the flow of material during said pressing step.

7. The method of claim 2, comprising sintering said body to a density of over percent of the theoretically possible density prior to said step of placing said body in said first die.

8. The method of claim 2, in which said body is comprised of powder mixtures of different alloying elements, comprising sintering said body to such a degree that the diffusion of the alloying elements takes place substantially during the sintering and at the latest during heating of the body to forging temperature prior to said step of placing.

9. The method of claim 8, wherein said sintering is performed at temperatures ranging from 1100 C to 1200 C, wherein the body is heated to a forging temperature of about 1 C to 1250 C, and wherein said step of pressing comprises applying drop forging pressure to said body above 10 tons per square centimeter.

10. The method of claim 2, wherein said step of pressing in a single step comprises pressing said body to have a density corresponding substantially to 100 percent of the theoretically possible density. 

1. A method for producing a drop forged sintered workpiece from a pre-pressed sintered body in which the body is deformed by upsetting to such a degree that the material of the body begins to flow; comprising placing a solid cylindricAl sintered prepressed body into a cavity of a forging die, the cavity having a lateral dimension normal to the forging direction that is larger than the diameter of said body, and substantially completely pressing the material of the sintered body in a single step out of the central axial portion thereof to flow into the circumferential region of said die.
 2. A method for producing a drop forged sintered workpiece from a pre-pressed sintered body in which the body is deformed by upsetting by such a degree that the material of the body begins to flow; comprising placing a first end of a solid cylindrical sintered pre-pressed body onto a central core of the cavity of a first forging die, the cavity having a lateral dimension normal to the forging direction and axis of the body that is larger than the direction of said body, and substantially completely pressing the material of said body in a single step out of the central axial portion thereof to flow into the circumferential region of said first die by pressing the other end of said body with a second forging die.
 3. The method of claim 2 for forming a gear, in which said cylindrical body has a flow cone at one end, and said circumferential region of said first die is formed with teeth for forming the teeth of said gear, wherein said step of placing comprises placing said body in said first die with said flow cone facing said first die, whereby said body will flow primarily radially outwardly and toward said second die in said pressing step.
 4. The method of claim 2, in which said second die has a central core with a cross section smaller than the cross section of the core of said first die, wherein said step of pressing the other end of said body comprises pressing the other end of said body with said central core of said second die.
 5. The method of claim 2, in which said central core comprises a punch in said first die displaceable into said second die, wherein said step of pressing comprises continuously pushing said punch into said cavity of said first die.
 6. The method of claim 2, comprising locally preheating said body prior to said pressing step whereby different portions of said body have different temperatures, to control the flow of material during said pressing step.
 7. The method of claim 2, comprising sintering said body to a density of over 80 percent of the theoretically possible density prior to said step of placing said body in said first die.
 8. The method of claim 2, in which said body is comprised of powder mixtures of different alloying elements, comprising sintering said body to such a degree that the diffusion of the alloying elements takes place substantially during the sintering and at the latest during heating of the body to forging temperature prior to said step of placing.
 9. The method of claim 8, wherein said sintering is performed at temperatures ranging from 1100* C to 1200* C, wherein the body is heated to a forging temperature of about 1100* C to 1250* C, and wherein said step of pressing comprises applying drop forging pressure to said body above 10 tons per square centimeter.
 10. The method of claim 2, wherein said step of pressing in a single step comprises pressing said body to have a density corresponding substantially to 100 percent of the theoretically possible density. 